The past few decades have seen many shifts in electronics and semiconductor packaging that have impacted the entire semiconductor industry. The introduction of surface-mount technology (SMT) and ball grid array (BGA) packages were generally important steps for high-throughput assembly of a wide variety of integrated circuit (IC) devices, while, at the same time, allowing reduction of the pad pitch on the printed circuit board. Conventionally packaged ICs have a structure basically interconnected by fine gold wire between metal pads on the die and electrodes spreading out of molded resin packages. Dual inline package (DIP) and quad flat package (QFP) are fundamental structures of current IC packaging. However, increased pin count peripherally designed and arranged around the package typically results in too short of a pitch of lead wire, yielding limitations in board mounting of the packaged chip.
Chip-scale or chip-size packaging (CSP) and BGA are just some of the solutions that enable dense electrode arrangement without greatly increasing the package size. CSP provides for wafer packaging on a chip-size scale. CSP typically results in packages within 1.2 times the die size, which greatly reduces the potential size of devices made with the CSP material. Although, these advances have allowed for miniaturization in electronic devices, the ever-demanding trend toward even smaller, lighter, and thinner consumer products have prompted even further attempts at package miniaturization.
To fulfill market demands toward increased miniaturization and functionality, wafer-level CSP (WLCSP) has been introduced in recent years for generally increasing density, performance, and cost-effectiveness, while decreasing the weight and size of the devices in the electronic packaging industry. In WLCSP, the packaging is typically generated directly on the die with contacts provided by BGA and bump electrodes. Recent advanced electronic devices, such as mobile phones, mobile computers, camcorders, personal digital assistants (PDAs), and the like, utilize compact, light, thin, and very densely packaged ICs. Using WLCSP for packaging smaller die size devices with lower numbers of pins, corresponding to larger number of chips on one wafer, is, therefore, usually advantageous and cost-effective.
One disadvantage of current WLCSP technology is the formation of cracks between the solder ball and the electrode post. The solder ball or bump is typically placed onto the bump electrode or post directly, relying on the soldered joint for structural integrity. The different layers making up the WLCSP device typically have different coefficient of thermal expansion (CTE). As a result, a relatively large stress derived from this difference is exhibited on the joint between the post and the bump electrode, which often causes cracks to form in the bonding area between the bump electrode/post and the solder ball or bump. Additionally, the solder ball is typically located above the layers of the underlying wafer. This exposure makes the solder ball more susceptible to physical impact, and places the more vulnerable joint in an exposed position as well.
FIG. 1 is a cross-section of a typical, single solder ball of WLCSP feature 10. WLCSP feature 10 in formed directly on die 100. Copper pad 102 is formed on die 100. Copper pad 102 acts as a contact and bonding pad for solder ball 101. During the soldering process, intermetallic compounds (IMC) are naturally formed in a layer, i.e., IMC formation layer 103, at the joint between solder ball 101 and copper pad 102. While existence of IMC formation layer 103 generally signifies a good weld between the solder and the substrate, it is usually the most brittle part of the weld. Because the weld joint is so small in WLCSP, cracks, such as crack 104, may form more easily under the stresses experienced at the joint, and such cracks, because of the size of the overall package, may be more damaging. Furthermore, IMC layer 103 is located above the top surface of die 100, thus, exposing this area to greater direct physical impact. A small crack that starts along one side of solder ball 101, such as crack 104, may easily propagate across the length of the straight solder joint.
One method that has been suggested to diminish this stress cracking is described in U.S. Pat. No. 6,600,234, to Kuwabara, et al., entitled, “MOUNTING STRUCTURE HAVING COLUMNAR ELECTRODES AND A SEALING FILM.” This method describes forming a sealing film using multiple layers where a portion of the bump electrode protruding from the sealing film. The protruding electrode assists in absorbing part of the stress caused by the difference in CTE. The multiple layers of the sealing film are also selected to have graduated CTE, such that the CTE of the film near the substrate is close to the CTE of the substrate, while the CTE of the film near the circuit substrate is close to the CTE of the circuit substrate. This graduated CTE helps alleviate the stresses that would be cause by sharply different CTE. However, the multiple layers of the sealing film still usually exhibit a weak sheer strength and do not reduce the propagation of any cracks that may form in the IMC layer, thus, reducing the overall reliability of the joint.
An additional method suggested to improve CSP is described in U.S. Pat. No. 6,717,245, to Kinsman, et al., entitled, “CHIP SCALE PACKAGES PERFORMED BY WAFER LEVEL PROCESSING.” This method provides for a first bumped layer that is completely encapsulated in an epoxy or other similar material. This encapsulation layer is then polished back to expose the tops of the encased bumps. Regulation solder balls are then printed or placed onto the exposed portions of the first bumped layer. By isolating the solder balls from the circuit board through the first encapsulated layer of bumps, the stress caused by thermal expansion is reduced. However, the ball joints still are subject to cracks that form along the IMC layer, thus again, reducing the overall reliability of the joint.
An additional method suggested to improve CSP is described in U.S. Pat. No. 6,906,418, to Hiatt, et al., entitled, “SEMICONDUCTOR COMPONENT HAVING ENCAPSULATED, BONDED, INTERCONNECT CONTACTS.” This method provides two different embodiments for CSP. The first embodiment extends the tip of an interconnect contact from the die pad through an insulating layer. After a metallization layer is deposited on the tip of the interconnect contact, solder balls are placed onto each of the extended tips. The metallization layer is selected to improve the bonding between the metallization material and the solder ball. However, the ball joint is still subject to considerable sheer forces because the ball joint is at or above the surface of the insulating layer. The second embodiment provides for the solder ball to be placed directly onto the die bonding pad or directly onto a redistribution layer. An insulating layer is then used to encapsulate the solder ball leaving a portion exposed for contact. While this embodiment improves the ball joint strength, placing the solder ball directly onto the die contact pads is a complex design process. The level of customization that would occur between the wafer fabrication process and the subsequent packaging process would greatly increase the costs of CSP features. Moreover, the configurations of interconnect contacts would also be limited to the die contact pad configuration.